The present invention relates to image sensors such as image pickup devices for use in television cameras. In particular, the present invention relates to an image pickup device of high sensitivity that is capable of picking up an image of a very dark object through electron image intensification.
Various proposals have been made to increase the sensitivity of image pickup devices and one approach consists of placing an image intensifier tube (hereinafter abbreviated as an I.I. tube) in front of the photosensitive surface of an image pickup device. The key concept of this design is that an image of an object is made much brighter by means of the I.I. tube and used as an input image for the image pickup device, thereby increasing the sensitivity of the device. To attain the intended result, the output image from the I.I. tube must be transmitted correctly to the photosensitive surface of the image pickup device. To meet this need, it is common practice to attach a fiber plate both to the output face of the I.I. tube and to the input face of the image pickup device.
FIG. 6 shows the construction of a color television camera in which an electric field (focusing) type I.I. tube 13 is coupled to a photoconductive storage camera tube (e.g. Saticon) 15 using a fiber plate 12a as a light-receiving surface, and this camera has proved to be 10-15 times as sensitive as ordinary color television Cameras (see Ohnishi and Yamashita, NHK Giken Gepppo, Vol. 24, No. 1, 1981). The other components of the camera shown in FIG. 6 are as follows: 2, a photocathode; 9, an electrode; 10, an insulator; 12b and 12c, a concave fiber plate each; 11, a fluorescent screen; and 14, a photoconductive surface. An I.I. tube has been coupled to other photoconductive camera tubes which employ different materials in the photosensitive layer than in Saticon, such as Chalnicon (see Inoue and Aihara, "Image Picking-up Characteristics of Chalnicon with Image Intensifier," Preprint 2--2 for the 1984 Annual Meeting of the Institute of Television Engineers of Japan) and Newvicon (see Yamamoto, "Newvicon with Fiber Plates," National Technical Report, Vol. 25, No. 2, 1979).
FIG. 7 shows an example of the case in which not only high sensitivity but also size reduction and elimination of image distortion could be realized by using a proximity (focusing) type I.I. tube 16 (see Kawamura and Yanagisawa, "Development of Proximity Photocathode Technology and Its Application to Image Intensifiers," the Journal of the Institute of Television Engineers of Japan, Vol. 36, No. 3, 1982). Other components of the system shown in FIG. 7 are as follows: 12d, a fiber plate; 1, a faceplate; 3, an anti-reflection layer; 4, a metal-back layer; 5, a phosphor layer. The members 3,4 and 5 combine to form a fluorescent screen 11. The other components which are the same as those shown in FIG. 6 are identified by like numerals.
If further enhancement of sensitivity is required, a plurality of I.I. tubes may be cascade-connected or a MCP (microchannel plate) may be incorporated in an I.I. tube to intensify the electron image.
A potential application of these techniques is in the effort to increase the sensitivity of solid-state image pickup devices. An example of this effort is shown in FIG. 8 and consists of coupling an I.I. tube 16 to a fiber plate 12a which is closely attached to the front face of the photosensitive layer 7 of a solid-state image pickup device 8. Instead of the I.I. tube 16, an electric field type I.I. tube as indicated by 13 in FIG. 6 may be coupled to the solid-state image pickup device 8. However, in order to fully exploit the small size and non-distortion feature of solid-state image pickup devices, it is more convenient to use the distortion-free proximity type I.I. tube 16 rather than the electric field type I.I. tube 13 which is prone to image distortion. Shown by 8a in FIG. 8 is a package for the solid-state image pickup device 8.
A version similar to that envisaged by the present invention is shown in FIG. 9, in which a solid-state image pickup device 8 such as CCD is sealed within an image tube 13 having a photocathode 2. Being named ICCD (intensidifed charge-coupled device), this system has claimed a sensitivity increased by a factor of 2,500 under an acceleration voltage of 20 kV but has not yet emerged from the laboratory (see J. L. Lowrance et al., "ICCD Development at Princeton," Adv. E.E.P., Vol. 52, pp. 441-452, 1979).
As described above, the prior art techniques for enhancing the sensitivity of image pickup devices through addition of I.I. tubes are not effective unless the focused output image on the I.I. tube is transmitted by some method to the photosensitive surface of an image pickup device in an optically correct manner. To meet this mandatory requirement, at least two fiber plates have been necessary as basic means for coupling an I.I. tube to an image pickup device. They are the fiber plate 12c or 12d on the output side of the I.I. tube 13 or 16 and the fiber plate 12a on the input side of the image pickup device 15 or 8 (see FIGS. 6, 7 and 8). The use of these fiber plates 12a, 12c and 12d will eventually cause severe deterioration in quality of an output image which is dependent on the characteristics of the fiber plates. The fiber plate is composed of a bundle of optical glass fibers that are worked into a plate form. The fiber plate has the advantage of permitting an optical image on the input surface to be transmitted to the output surface with minimum optical loss but, at the same time, it has the following disadvantages.
(1) Despite comparatively low optical loss, fiber plates with the usual thickness of 5-10 mm do not have a transmittance exceeding about 70%, so the overall transmittance of two fiber plates is about 50%, thus reducing the image intensification of the system by one half.
(2) Image transmission through a fiber plate involves sampling by individual optical fibers and this leads to deteriorated image resolution. A combination of two fiber plates will cause another problem, namely, image interference such as beat or moire.
(3) Fiber plates suffer from several optical defects that are introduced during fabrication and to name a few: failure to transmit light on account of breakage or devitrification of individual fibers; flaws due to uneven distribution of absorbers that are inherent in fiber structure; nonuniformities called "block lines" or "chicken wires" that are introduced when a plurality of multi-fibers, each including a certain number of fibers, are further bundled together; distortions caused by twisting or shifting of multi-fibers; and shading which occurs on account of the difference in transmittance between individual multi-fibers. These many optical defects are unwanted since they all deteriorate the quality of output image.
(4) These optical defects could be avoided to some extent by selecting fiber plates of good quality, but then such fiber plates are very expensive since their production yield is low.
Thus, the conventional techniques for increasing the sensitivity of image pickup devices by coupling them with I.I. tubes via fiber plates have suffered from the defects inherent in fiber plates and the biggest problem with the prior art has been how to prevent image deterioration that would otherwise result from sensitivity loss, reduced resolution, and flaws or nonuniformities on the screen.
With a view to solving this problem, the present inventors previously conducted an experiment making an attempt to eliminate one fiber plate by bonding the fiber plate 12d on the output surface of an I.I. tube 16 (see FIG. 10) to a solid-state image pickup device 8 by means of an adhesive. This technique proved to be more effective in improving image quality than the previous methods of using two fiber plates 12a and 12d as shown in FIGS. 7 and 8, but the results were still unsatisfactory. Furthermore, this approach is not recommendable from the viewpoint of workmanship if a camera tube is to be used as the image pickup device.
The system shown in FIG. 9 is a refinement of this approach, in which a solid-state image pickup device 8 such as CCD is sealed within an image tube 13 having a photocathode 2. Unfortunately, however, this improved version has not yet been commercialized, possibly for the following reason: The alkali metals, which were used in forming the photocathode with the image tube 13 being evacuated, will deposit on the photosensitive surface 7 of the CCD enclosed with the image tube 13, and this will either cause an immediate drop in resolution or slowly deteriorate the performance of the CCD. Thus, the operating life of the system may be long enough to enable acquisition of experimental data but is too short to guarantee its commercial use.